1. Technical Field
The present invention relates to a filter circuit used in a wireless communications device or the like and an impedance circuit, such as a matching circuit, and more particularly to an impedance circuit capable of operating in response to more than one frequency band, and a filter circuit, an amplifier circuit, a semiconductor integrated circuit, an electronic component, and a wireless communications device using such an impedance circuit.
2. Background Art
A system that uses switching over more than one frequency band has recently become known in a communications system. For example, in the wireless LAN method, the system switches from a 2.4 GHz band to a 5 GHz band and vice versa, or in the GSM method, the system switches from a 900 MHz band to a 1.8 GHz/1.9 GHz band and vice versa.
In a communications system configured to switch over more than one frequency band as described above, for the switching to be enabled, it is common to establish multiple signal paths for radio frequency bands in an RF portion, so that the signal paths are switched depending on the radio frequency band to be used.
For example, in the case of a filter circuit, as is shown in FIG. 21, a filter circuit 600 comprises filters 51 and 52 suitable for radio frequencies f1 and f2 in respective radio frequency bands, and switch circuits SW51 and SW52 provided respectively to the input end and the output end of these filters 51 and 52. The signal paths are switched by enabling either the filter 51 or 52 with the use of the switch circuits SW51 and SW52.
Also, for example, as is shown in FIG. 22, an amplifying element portion forming the RF portion is provided with amplifying elements 1101a and 1101b suitable for radio frequencies in respective radio frequency bands, and input matching circuits 1102a and 1102b and output matching circuits 1103a and 1103b are disposed before and after the amplifying elements 1101a and 1101b, respectively. By switching over a path for a first frequency signal f1 formed from the input matching circuit 1102a, the amplifying element 1101a, and t-he output matching circuit 1103a, and a path for a second frequency signal f2, formed from the input matching circuit 1102b, the amplifying element 1101b, and the output matching circuit 1103b, with the use of an input end change-over switch 1104 and an output-end change-over switch 1105 provided at an input end and an output end of the paths, respectively, a processing path is switched to either path suitable for the signal f1 or f2.
This is because it is practically difficult to achieve a circuit capable of attaining a satisfactory characteristic across all the usable bands, as respective circuits forming the RF portion. Even when a circuit is configured to attain a homogeneous characteristic across all the bands, the performance of such a circuit markedly deteriorates compared with a case where a circuit is formed separately for each usable frequency band. For this reason, it is common to form a circuit separately for each, frequency band.
However, in a case where a signal path forming the RF portion is provided separately for each frequency band to be used as described above, the number of components and an occupied area are increased, which makes it difficult to reduce the cost and the size of the circuits.
As a method of avoiding such problems, there have been proposed various methods by which a signal path forming the RF portion is shared by all the frequency bands.
For example, as a method of sharing a filter circuit forming the RF portion by more than one band, there has been proposed a method, by which a varactor diode is used in a resonant circuit to make a capacity variable with a control voltage supplied from the outside, so that a frequency band allowed to pass is changed by adjusting the resonance frequency (see, for example, Japanese Unexamined Patent Publication No. Hei-7-321586).
Also, as a method of sharing a matching circuit used for an amplifier forming the RF portion by more than one band, there has been proposed a method, by which an independent amplifying element is used, and matching circuits, as those at the input end and output end of the amplifier, are provided for each frequency, so that the system is able to respond to more than one frequency band by switching to the matching circuits corresponding to a frequency to be used, with the use of switches (see, for example, Japanese Unexamined Patent Publication No. Hei-5-37255).
In the case of a filter circuit, however, when a frequency band allowed to pass is changed by adjusting the resonance frequency as described above, there arises a need to use a low-Q element, such as a varactor diode, in the resonant circuit, which poses a problem in that the filter characteristic is deteriorated.
Also, when the matching circuits are switched to correspond to the frequency to be used as described above, the number of matching circuits increases with the number of usable frequencies. In particular, in terms of the area occupied by the circuits, the input and output matching circuits occupy a larger area than the active element portion, which poses a problem in that this configuration is less effective in achieving a fundamental reduction of the circuit size.
In addition, when the matching circuit is a matching circuit covering a broad band including usable frequencies and the circuit to be matched is an amplifier, a signal in a frequency band other than the desired frequency band is also amplified, which makes this configuration disadvantageous in terms of efficiency of the amplifier.
The invention was devised in view of the conventional problems that remain unsolved, and therefore relates to an impedance circuit capable of operating in response to more than one frequency band without deteriorating the characteristic or increasing the circuit size, and to a filter circuit, an amplifier circuit, a semiconductor integrated circuit, an electronic component, and a wireless communications device using such an impedance circuit.